The technical field generally relates to selective catalytic reduction (SCR) systems having an NH3 sensor included as a control element. A failure in the NH3 sensor may significantly impact the control of an SCR system relying upon the NH3 sensor. Because the system relies upon the NH3 sensor, in certain applications a failure of the sensor must be detected, and/or the impact of the lack of a sensor failure detection must be accounted for in an estimate of the impact of emissions of the system including the sensor. Detection of a failed NH3 sensor, including at a position downstream of a reductant injector and at least a portion of the total SCR catalyst present in the system, is challenging. Therefore, further technological developments are desirable in this area.